Hello,

Star magnolia, (Magnolia stellata), https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=282531&isprofile=1&basic=star%20magnolia

Is susceptible to leaf spot caused by insects, bacterial or viral diseases. However, the most common cause of leaf spot is caused by four common fungi. These fungi may cause leaf spot on other ornamental trees also. When environmental conditions are moist and the spores of these fungi are present, they can germinate on the wet leaves of many different plants. Therefore, the disease is likely to occur during rainy or humid weather.

Leaf spot is not considered a threat to a healthy tree. But it can be serious to trees under stress, such as young trees. It can be controlled by cultural practices. Because the fungus survives in compost and soil, it is best to rake up and dispose of any leaves or branches that have fallen to prevent any future infection. Increase air circulation around the tree by pruning out broken and rubbing branches and branches of neighboring trees that may encroach on the tree’s space. At the end of the growing season, prune out any infected stems and leaves and clean up and dispose of the infected material. Read more here: https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#identification-1156261

If the leaf spot is serious enough to warrant chemical intervention, it is necessary to identify the fungus causing the problem. MSU Extension’s Plant and Pest Diagnostic Lab can identify the fungus. Go to: https://www.canr.msu.edu/pestid/ to learn how to submit a sample.

Because fungicides are a preventative, not a cure, it is necessary to spray the plant before the fungus attacks the leaves. When using chemicals, read the label to ensure that it can be used against that disease on that specific plant. Follow the label directions on how to use the chemical.

A certified arborist can assess the tree. To find an arborist in your area, go to www.treesaregood.org, the website of the International Society of Arboriculture.

Thank you for contacting Ask Extension.